Optical parametric oscillators and second harmonic generators are commonly used nonlinear optical devices, and the second order nonlinear optical material is the key material for achieving the function thereof. For a second order nonlinear optical material in the infrared region, this disclosure uses a monoclinic phase Ga2S3 crystal which has never been used in the art, obtaining an optical parametric oscillator and a second harmonic generator having high laser damage threshold.
Because of the application of laser technology, the nonlinear optical crystal materials have drawn increasing attention of the public. In particular, the second order nonlinear optical crystal materials of deep ultraviolet and middle/far infrared regions, which are far from satisfying the requirements of the applications due to the lack of varieties, have become the hot spot of research. The chalcogenide system has become a direction of the research of middle/far infrared second order nonlinear optical crystal, wherein, for example, AgGaS2 (AGS), AgGaSe2 (AGSe), AgGa(1-x)InxSe2, GaSe, LiInS2 (LIS), LiInSe2, and the like have drawn wide attention. These chalcogenides are mainly ternary compounds or more. Less attention is paid to the research of the second order nonlinear optical properties of binary chalcogenide compounds. However, comparing with the ternary compounds or more, the binary chalcogenides generally have the advantages of simple structure, convenient synthesis, stable physical and chemical features, and so on.
Ga2S3 has three crystal phases: monoclinic phase (Cc), hexagonal phase (P63mc) and cubic phase (F-43m), which are all noncentrosymmetric space groups, meaning that Ga2S3 may have second order nonlinear optical effects. In 1961, Goodyear et al. reported the monoclinic phase structure (Cc) of Ga2S3 in Acta Crystal for the first time. There is no report regarding using Ga2S3 as the infrared second order nonlinear optical materials hitherto.
There are two known synthetic methods for Ga2S3, both of which employ Ga and S elementary materials as the starting reactants. The first method comprises mixing Ga and S in a proper ratio, enclosing into a evacuated quartz tube, keeping at 450° C. for 5 days, and then heating at a rate of 50° C./12 h to a temperature of 1100° C., and naturally cooling down to obtain polycrystalline powders of Ga2S3. The second method comprises placing equal amount of Ga and S into two quartz boats in a flame-sealed quartz tube under a vacuum condition, heating the quartz boat containing Ga to 1150° C., and heating the quartz boat containing S to 450-500° C. After one day, polycrystalline powders of Ga2S3 are formed on one end of the quartz boat containing Ga. The Ga2S3 obtained in both methods are of a monoclinic phase. Embodiments of the disclosure use Ga2O3, S powder, and B powder as the starting materials, and synthesize monoclinic phase Ga2S3 employing a high temperature solid-state boron-sulfur method.